Position-dependent compliance compensation in a machine tool

ABSTRACT

The invention relates to a method and a device for position-dependent compliance compensation in a machine tool. To permit the lowest possible machining time or greatest possible contour fidelity in the machining of a workpiece with a machine tool, the following method steps are proposed:—deriving the compliance ( 2 ) of the machine tool ( 1 ) at a position of a tool of the machine tool ( 1 ) from machine data ( 3 ) stored in a memory,—determining a machining force ( 4 ) acting on the tool during a machining process at said position,—modification of at least one machining parameter ( 5 ) that has an influence on the machining process for the said position in dependence on the derived compliance ( 2 ) and the machining force ( 4 ) in such a way that a displacement ( 6 ) of the tool with respect to a desired position that is caused by the compliance ( 2 ) of the machine tool ( 1 ); and the machining force ( 4 ) is counteracted.

The invention relates to a device and method for compensating a machinetool's position-dependent compliance, to a machine tool that includes adevice of said type, and to a computer program for implementing a methodof the cited kind.

A compromise between as short as possible machining time and as high aspossible finishing accuracy often has to be accepted when workpieces aremachined, in particular cut, by means of a machine tool. The productionprocess is basically designed such that a minimally required finishingaccuracy will be maintained everywhere on the workpiece being machinedand at all times during machining. Given these boundary conditions,maximum possible feed rates and cutting parameters must be targeted forthe machine tool in order to ensure as economical as possibleproduction.

Said maximum possible parameters depend substantially on the machinetool used. For example, parameters such as feed rates, cutting depths,tools employed etc. and the process forces resulting therefrom willimpact substantially on the achieved contour fidelity depending on themachine tool's rigidity.

The machine's rigidity is as a rule dependent on the current positioningof the machine tool's axes or, expressed another way, on the tool'scurrent position within the machine's operating space. Rigidity or, asthe case may be, its reciprocal, compliance, will hence vary during amachining operation during which what is termed the tool center point(TCP) moves within the operating space. Clearly visible contourdeviations can consequently occur while a workpiece is being machined ifconstant parameters for feed rate, cutting depth and spindle speed areused during machining. A pre-specified contour fidelity can nonethelessbe achieved by dimensioning the cited machining parameters takingaccount of the machine tool's greatest compliance.

The object of the invention is to enable as short as possible machiningtime or as great as possible contour fidelity while a workpiece is beingmachined by means of a machine tool.

Said object is achieved by means of a method for compensating a machinetool's position-dependent compliance having the following method-relatedsteps:

-   -   deriving the machine tool's compliance at a position of a tool        of the machine tool from machine data stored in a memory,    -   determining a machining force acting upon the tool during a        machining process at said position,    -   modifying at least one machining parameter affecting the        machining process for said position as a function of the derived        compliance and the machining force in such a way that a        displacement of the tool with respect to a setpoint position due        to the machine tool's compliance and the machining force will be        counteracted.

The object is further achieved by means of a computer program forimplementing said method.

The object is furthermore achieved by means of a device for compensatinga machine tool's position-dependent compliance having:

-   -   a memory for machine data,    -   deriving means for deriving the machine tool's compliance at a        position of a tool of the machine tool from the machine data,    -   determining means for determining a machining force acting upon        the tool during a machining process at said position,    -   modifying means for modifying at least one machining parameter        affecting the machining process for said position as a function        of the derived compliance and the machining force in such a way        that a displacement of the tool with respect to a setpoint        position due to the machine tool's compliance and the machining        force will be counteracted.

The object is furthermore achieved by means of a machine tool having adevice of the cited kind.

The invention is based on the knowledge that a machine tool's machiningtime can be significantly reduced while at the same time maintaining apre-specified contour fidelity if machining parameters determining themachining process are varied as a function of the machine tool'scompliance at a specific position of the tool during the machiningprocess. By taking account of the machine tool's position-dependentcompliance during the machining process it is possible to significantlyreduce the displacement of the tool's tool center point and hence thedeviation from a pre-specified contour as compared with present-daymachine tools in the case of which the position dependency of themachine tool's compliance is not taken into account.

The machine tool's compliance is first derived for a specific positionof the tool based on the machine data so that at least one of saidmachining parameters can be modified in a targeted manner. The machinedata can be, for example, a three-dimensional machine model from whichthe machine tool's rigidity or compliance as a function of the positioneither proceeds directly or can be derived therefrom. If the complianceat said position is known, then the machining force acting upon the toolat said position will furthermore be determined. The machining force canbe determined in advance through simulation and/or calculation, or itcan alternatively be ascertained using measuring techniques.

If the force and compliance at said position are known, it will bepossible to determine a notional displacement of the tool with respectto a setpoint position thereof that can be counteracted through suitablymodifying at least one of the impacting machining parameters. Thenotional displacement would occur without any modifying of the machiningparameters.

In an advantageous embodiment of the invention, the machine tool isprovided for cutting operations. Cutting operations require adherence tominimum requirements placed on contour fidelity, something that is madevery difficult because of wide variations in compliance within themachine tool's operating range and will be at the expense of operatingtime if the position dependency of the machine tool's compliance doesnot impact on the machining parameters. The machining parameter(s) willbe modified compliance-dependently during the machining process by theinventive method, the result of which will be a substantial reduction inmachining time.

Various values are possible for the position-dependent and hencecompliance-dependent machining parameter requiring to be modified. In anadvantageous embodiment variant of the invention, the machiningparameter is a setpoint value for the tool position that ispre-specified by a control program, in particular an NC program. Adisplacement due to changing machine tool compliance can be effectivelycounteracted by changing the setpoint value for the tool position.

As an alternative thereto, an advantageous embodiment variant of theinvention is characterized in that the machining parameter is a feedrate value or a cutting depth or a spindle speed.

Position-dependently and compliance-dependently modifying a plurality ofdifferent machining parameters during the machining process is of coursealso advantageous and encompassed by the invention.

There are also several possibilities for obtaining the machine dataserving as the basis for deriving the individual compliance valueswithin the machine's operating space. One advantageous embodimentvariant of the invention is characterized in that the machine data isobtained by simulating the machine tool's rigidity characteristics. Formachine tools there are generally three-dimensional machine models thatcan already be produced at the machine tool's design stage and fromwhich rigidity values of said kind can be ascertained. Finite elementsimulations are typically performed therefor.

Another advantageous embodiment of the invention is characterized inthat the machine data is ascertained using measuring techniques. Forincreasing accuracy it is also conceivable for the machine data to beascertained using both a measuring technique and a simulation technique.

In another advantageous embodiment of the invention, the machine dataincludes discrete compliance values at discrete points within themachine tool's operating space.

In another advantageous embodiment of an embodiment variant of saidkind, the machine tool's compliance at said position is derived byinterpolating the compliance values. For example, linear extrapolatingwill also make it possible to ascertain compliance values for positionswithin the machine tool's operating space that are not included in themachine data stored in the memory.

There are also several possibilities for determining the machiningforce. For example, one advantageous embodiment of the invention ischaracterized in that the machining force is measured by means of aforce sensor.

An advantageous alternative or additional embodiment variant of theinvention is one in which the machining force is determined by measuringdrive currents of the machine tool and a calculation of the machiningforce is determined from the measured drive currents.

In another advantageous embodiment of the invention, the machining forcecan furthermore be determined by performing a process simulation of themachining process.

In an advantageous embodiment of the invention, the machining parametercan be modified before a workpiece is machined. The position-dependentvalues for the machine tool's compliance and the machining force musttherein be ascertained in advance in order, for example, toappropriately modify in advance an NC program provided for controllingthe machining process. For example, setpoint values determining thecontour curve can in that way be adjusted ahead of the machining processtaking the position-dependent compliance into account.

An advantageous alternative or additional embodiment variant of theinvention is one in which the machining parameter is modified while aworkpiece is being machined. The compliance values within the machine'soperating space are therein expediently also first determined in advanceby means of simulating or measuring. The machining force acting upon thetool while the workpiece is being machined is, though, determined onlineso that during the machining process a displacement of the tool centerpoint ensuing therefrom can be calculated that is counteracted likewiseonline by suitably modifying the machining parameter.

The invention is described and explained in more detail below withreference to the exemplary embodiments shown in the figures, in which:

FIG. 1 is a schematic representation of a method for compensating amachine tool's position-dependent compliance and

FIG. 2 shows a method for compensating a machine tool'sposition-dependent compliance with a machining force being calculatedfrom measured drive currents.

FIG. 1 is a schematic representation of a method for compensating aposition-dependent compliance 2 of a machine tool 1. The compliance 2 iscompensated as a function of the tool within the operating space of themachine tool 1 through online adjusting of machining parameters 5 thatimpact on the contour requiring to be traveled along during themachining process. For each point P through which the tool travelsduring the machining process a compliance matrix 2 is ascertained usingmachine data 3 stored in a memory. The compliance matrix 2 can beascertained online during the machining process or even ahead of themachining process so that the values will already be available when theworkpiece is machined.

A machining force 4 is furthermore determined in the form a machiningvector F for each position P requiring to be assumed by the tool. Fromthe compliance matrix G and machining vector F for the current point Pit is finally possible to determine a displacement d of the workpieceor, as the case may be, its tool center point by multiplying saidvariables. The aim of compensating is to keep said displacement 6 assmall as possible. For that purpose it is ascertained via a suitablemodification function 10 how one or more machining parameters 5 are tobe modified in order to keep the displacement 6 as small as possible orto ensure that the displacement 6 will not exceed a pre-specifiedmaximum value. The machining parameters 5 are, for example, a setpointvalue, pre-specified in the NC program 7, for the tool position, thecurrent cutting depth, a current value for the feed rate, or the spindlespeed of the machine tool 1.

The special feature of the method being presented is that the machiningparameters 5 are adjusted online as a function of the compliance of themachine tool 1 and of the force 4 acting upon the tool. A minimummachining time can be achieved thereby if the contour fidelity has beenpre-specified or the contour fidelity improved thereby if the time isconstant.

FIG. 2 shows a method for compensating a position-dependent compliance 2of a machine tool 1 with a machining force 4 being calculated frommeasured drive currents 8. One scenario for using integratedposition-dependent compliance compensation of said kind is as follows:

The compliance 2 in the form of a compliance matrix can be ascertainedfor each position P online during the machining process from machinedata 3 present in the form of a rigidity model, determined once inadvance, of the operating space of the machine tool 1. The model fordetermining the position-dependent compliance 3 is by contrastdetermined independently of subsequent machining once only specificallyfor the relevant machine tool and its configuration by means of, forexample, simulation or measuring of the entire operating space.

For machining a workpiece the operator loads a desired control programin the form of an NC program. In the control of the machine tool 1 theoperator furthermore activates an instruction that activates theposition-dependent compliance compensation. Alternatively, theposition-dependent compliance compensation is invoked automatically whenthe NC program is launched.

The control then executes the NC program taking the machine complianceinto account. That takes place as follows for different points P towhich the tool travels:

With the aid of the model for determining the position-dependentcompliance 3, the compliance matrix 2 is first determined for theposition P that is traveled to. The drive currents 8 consisting of threeaxle currents and one spindle current are furthermore ascertained atsaid operating point P through current measuring. Using a process forcemodel 9 it is possible to ascertain the forces 4 acting upon the tool 1at the current position P from the measured drive currents 8. The resultis three force components F_(X), F_(y), F_(z) for the three spatialdimensions. A displacement of the tool relative to all three spatialdimensions can then be determined from the force vector F resultingtherefrom and from the determined compliance matrix 2.

A check is then carried out to determine whether the displacement 6exceeds a pre-specified maximum value either in terms of amount orrelative to one of the three spatial dimensions. If it does, amodification function 10 will be activated that will modify at least onemachining parameter 5 in such a way that such kind of displacement 6 ofthe tool or, as the case may be, tool center point can be counteracted.Modifying can be performed by, for example, changing the NC programrequired for controlling. Alternatively, the setpoint drive values canalso be modified directly. Instances of machining parameters 5 requiringto be modified are the feed rate of the machine tool 1, the cuttingdepth, the spindle speed, and setpoint values, initially pre-specifiedin the NC program, for the contour along which the tool is to travel.

As an alternative to what has been shown, namely calculating themachining force 4 from the drive currents 8, it is also conceivable andencompassed by the invention to ascertain the machining force 4 directlyusing measuring techniques. The overall calculating time for performingcompliance compensation can in that way be somewhat reduced and theaccuracy of the force values possibly improved.

1.-28. (canceled)
 29. A method for compensating a position-dependentcompliance of a machine tool, comprising the steps of: deriving fromstored machine data the position-dependent compliance of the machinetool at a tool position of a tool of the machine tool, determining amachining force acting upon the tool during a machining process at thetool position, and modifying, depending the derived compliance and thedetermined machining force, at least one machining parameter affectingthe machining process at the tool position so as to counteract adisplacement of the tool from a desired tool position caused by thecompliance of the machine tool and the machining force.
 30. The methodof claim 29, wherein the machine tool is provided for cuttingoperations.
 31. The method of claim 29, wherein the at least onemachining parameter is a value of the desired tool position specified ina control program.
 32. The method of claim 31, wherein the controlprogram is a NC program.
 33. The method of claim 29, wherein the atleast one machining parameter is a feed rate value or a cutting depth ora spindle speed.
 34. The method of claim 29, wherein the machine data isdetermined from a simulation of a rigidity characteristic of the machinetool.
 35. The method of claim 29, wherein the machine data aredetermined from a measurement.
 36. The method of claim 29, wherein themachine data include discrete compliance values at discrete locationswithin a workspace of the machine tool.
 37. The method of claim 36,wherein the compliance of the machine tool at an arbitrary location isderived by interpolating between the discrete compliance values.
 38. Themethod of claim 29, wherein the machining force is measured with a forcesensor.
 39. The method of claim 29, further comprising the steps of:measuring drive currents of the machine tool, and determining themachining force from the measured drive currents.
 40. The method ofclaim 29, further comprising the steps of: performing a processsimulation of the machining process, and determining the machining forcefrom the simulation.
 41. The method of claim 29, wherein the at leastone machining parameter is modified before machining of a workpiece. 42.The method of claim 29, wherein the at least one machining parameter ismodified during machining of a workpiece.
 43. A device for compensatinga position-dependent compliance of a machine tool, comprising: a memorystoring machine data, deriving means for deriving from stored machinedata the position-dependent compliance of the machine tool at a toolposition of a tool of the machine tool, determining means fordetermining a machining force acting upon the tool during a machiningprocess at the tool position, and modifying means for modifying,depending the derived compliance and the determined machining force, atleast one machining parameter affecting the machining process at thetool position so as to counteract a displacement of the tool from adesired tool position caused by the compliance of the machine tool andthe machining force.
 44. The device of claim 43, wherein the machinetool is a cutting machine tool.
 45. The device of claim 43, wherein theat least one machining parameter is a value of the desired tool positionspecified in a control program.
 46. The method of claim 45, wherein thecontrol program is a NC program.
 47. The device of claim 43, wherein theat least one machining parameter is a feed rate value or a cutting depthor a spindle speed.
 48. The device of claim 43, wherein the machine datais determined from a simulation of a rigidity characteristic of themachine tool.
 49. The device of claim 43, wherein the machine data aredetermined from a measurement.
 50. The device of claim 43, wherein themachine data include discrete compliance values at discrete locationswithin a workspace of the machine tool.
 51. The device of claim 50,wherein the deriving means are configured to derive the compliance ofthe machine tool at an arbitrary location by interpolating between thediscrete compliance values.
 52. The device of claim 43, wherein thedetermining means include a force sensor for measuring the machiningforce.
 53. The device of claim 43, wherein the determining meanscomprise a current sensor measuring a drive current of the machine tooland a calculating unit calculating the machining force from the measureddrive current.
 54. The device of claim 43, further comprising asimulator performing a process simulation of the machining process,wherein the determining means determine the machining force from theprocess simulation.
 55. The device of claim 43, wherein the modifyingmeans modify the at least one machining parameter before machining aworkpiece.
 56. The device of claim 43, wherein the modifying meansmodify the at least one machining parameter during machining aworkpiece.
 57. A machine tool comprising a device for compensating aposition-dependent compliance, the device comprising: a memory storingmachine data, deriving means for deriving from stored machine data theposition-dependent compliance of the machine tool at a tool position ofa tool of the machine tool, determining means for determining amachining force acting upon the tool during a machining process at thetool position, and modifying means for modifying, depending the derivedcompliance and the determined machining force, at least one machiningparameter affecting the machining process at the tool position so as tocounteract a displacement of the tool from a desired tool positioncaused by the compliance of the machine tool and the machining force.58. A computer program which enables a computer, after the computerprogram is loaded into a memory of the computer, to execute a method forcompensating a position-dependent compliance of a machine tool, themethod comprising the steps of: deriving from stored machine data theposition-dependent compliance of the machine tool at a tool position ofa tool of the machine tool, determining a machining force acting uponthe tool during a machining process at the tool position, and modifying,depending the derived compliance and the determined machining force, atleast one machining parameter affecting the machining process at thetool position so as to counteract a displacement of the tool from adesired tool position caused by the compliance of the machine tool andthe machining force.